Electro-optic modulators use an electric field to change the index of refraction of the substrate or medium where the beam of light (or laser) is found. Electro-optic modulators are optical devices which allow control of the power, phase, or polarization of a beam of light with an electrical control signal applied via a capacitor, p-i-n diode, or JFET, for example. The functionality is based on an electro-optic effect, that is the modification of the refractive index of a medium (e.g., crystal substrate) by an electric field in proportion to the field strength. Electro-optic devices can use free carrier dispersion to produce the refractive index change. In semiconductor materials, such as Silicon, the refractive index of light may be varied by varying the charge carrier concentration along the optical path. For example, when an electric potential is supplied to a Silicon P-N junction to forward bias a diode, the doped regions inject charge carriers into the Silicon depletion region. By free carrier dispersion, the change in the concentration of free charge carriers alters the refraction index.
Up to now the devices used to change the concentration of free charge carriers have been capacitors and p-i-n/p-n diodes. The majority of electro-optic modulators make use of the injection and/or depletion of free carriers, and the associated free carrier dispersion, within a region of the optical waveguide in which light is confined. Two types of two-terminal electrical devices have predominantly been used to modulate the density of free carriers, these being metal-oxide-semiconductor (MOS) capacitors, and p-i-n/p-n (diodes). The primary drawback of the MOS capacitor configuration is the limited overlap between the approximate 10 nm thick depletion/accumulation/inversion layer (where the concentration of free carriers is modulated by the gate voltage), and the optical mode, having a spatial extent of approximately 500 nm×500 nm, typically. While the p-i-n diode device geometry has been designed to dramatically improve upon the optical overlap issue, several other problems exist.
The p-i-n diode modulator can be used in two primary modes of operation. The first, in which the diode is first forward biased to inject minority carriers into the depletion region/waveguide core, and is then reverse biased to sweep out these carriers, is intrinsically slow. In forward bias, while forward current is flowing, the charge density within the waveguide takes a much longer time to reach steady state in comparison with when the diode is reverse biased. This results from the slow dynamics of minority carriers in silicon, and limits the fundamental modulation frequency at which an optical modulator/switch can be driven.
The second mode of operating a p-i-n diode modulator is in reverse bias depletion mode only. In this case, the diode is never forward biased to inject carriers, and only the concentration of the existing majority carriers (from dopants, thermal generation, etc.) is modulated within the diode depletion region/waveguide core. While the reverse bias only mode of operation enables intrinsically much faster modulation than permitted by the forward-reverse mode, the magnitude of modulation of the free carrier concentration is approximately ten times smaller, implying a ten times smaller change in the refractive index caused by free carrier dispersion. Therefore, p-i-n diode modulators operated in reverse bias only mode must be approximately ten times as long in order to attain the same modulation depth. The device footprint is thus adversely affected in order to obtain high speed modulation, often requiring the usage of traveling wave electrodes, further complicating the design.
Prior art two-terminal p-i-n/p-n diode modulators have the limitations that in forward-reverse bias operation, the forward biased diode turn-on is slow, restricting operation at higher speeds.
In reverse bias only operation, there is no introduction of excess carriers as in forward bias case, leading to the limitation that the net change in carrier concentration and waveguide refractive index is small. An additional solution to these problems is therefore required, in order to enable high frequency modulation/switching, within an ultra-compact device footprint.